A solar thermal power plant based on Direct Steam Central Receiver (DSCR) includes a large field of heliostats and a solar receiver placed on a tower of substantial height. The heliostats focus direct sunlight on to the solar receiver to produce steam to be utilized to run a steam turbine from producing electricity. Typically, the solar thermal power plant operates on a daily cycle, during clear sunlight hours, while shutting down in nights or in cloudy seasons. However, if the solar thermal power plant is to meet increasing electricity demand, it needs to be operable irrespective of the availability of solar light, i.e. in nights or in cloudy seasons. A realization of such a solar thermal power plant generates a requirement of storing solar thermal energy during day times and utilizing thereto in nights or in cloudy seasons. For such requirement, a central receiver including a solar energy storage fluid, such as molten salt, is generally used. The central receiver with molten salt is generally known as Molten Salt Central Receiver (MSCR).
A typical MSCR system 10 is evident in FIG. 1. The MSCR system 10 includes a MSCR 12, hot and cold storage tanks 14, 16 and a Molten Salt Steam Generator (MSSG) cycle 18. The molten salt fluid heated at the MSCR 12 is stored in the hot storage tank 14, at temperature of about 565° C., and after thermal energy thereof is being utilized by the MSSG cycle 18, it is stored in the cold storage tank 16, at temperature of about 290° C., from where it is further sent to the MSCR 12 to be reheated. The MSSG cycle 18 includes a steam generator arrangement 20, a reheat 22 and a turbine 24. The steam generator arrangement 20 utilizes the heat of the hot molten salt and converts feed water from a feed water tank 26 in to steam and send it to a high pressure turbine inlet of the turbine 24 for the conversion of heat to electricity through a generator ‘G’. Further, the steam from a high pressure turbine outlet of the turbine 24 is reheated by the reheat 20 by utilizing the hot molten salt. This reheated steam is supplied to an intermediate pressure turbine inlet of the turbine 24 for the conversion of heat to electricity.
Temperature and pressure of steam in the MSSG cycle 18 are generally limited, respectively, by the hot molten salt temperature, typically at 545° C., and by a pinch limitation in the MSSG cycle 18, typically at or lower to 115 bars. The pinch limitation in MSSG cycle 18 is dependent on the two important factors. Firstly, feed water temperature for being converted into steam shall need to be maintained above 240° C. to avoid freezing of the molten salt. Secondly, after the steam from a high pressure turbine outlet of the turbine 24 is reheated by the reheat 20 by utilizing the hot molten salt, the temperature of such used hot molten salt is still above the temperature at which it can be sent to the cold storage tank 14, i.e. above 290° C. A possible solution to avoid sending such hot molten salt from the reheat 20 is to mix it at any stage of the steam generator arrangement 20, i.e., between any of the two of economizer, superheater and evaporator. Due to these two pinch limitation requirement, low steam pressure is resulted as mentioned, which has various negative impact on the efficiency of the solar thermal power plant, including but not limited to, lower efficiency of the power plant, formation of cold spots/stagnation in-turn resulting freezing of molten salt, plugging and damage of heat exchange of the steam generator arrangement 20.
It is not that sufficient efforts have not been taken till date to resolve these pinch limitation requirement. One particular effort may be evident in FIG. 2, where the reheat 20, as shown in FIG. 1, has been removed, eliminating the need of reheating the steam that exits from the high pressure turbine outlet of the turbine 24. Such arrangement may avoid mixing of still hot molten salt at any stage of the steam generator arrangement 20, thereby precluding the requirement of pinch limitation. However, in this case the steam pressure provided at a high pressure turbine inlet of the turbine 24 should be increased enough to compensate for the efficiency lost by not carrying out the reheat. But due to low steam pressure, such arrangement may incur in very high wetness at an exit of the low pressure steam turbine, with a negative impact on efficiency and erosion of last stage blades.